1. Field of the Invention
The present invention relates to a multiple light beam scanning optical system such as an electrophotography apparatus for forming an image by scanning light beams.
2. Description of the Related Art
Heretofore, there has been used a light beam scanning optical system as an image writing device in an electrophotography process and mounted on a laser printer, a laser FAX or the like which is an output apparatus of computer and facsimile. Recently, in order to further raise a printing speed and a resolution, a demand for a scanning optical system using a plurality of light beams is increasing. A two-light-beam scanning optical system for scanning two beams meets with such demand. The two-light-beam scanning optical system according to the related art will be described hereinafter.
FIG. 10 is a perspective view showing a two-light-beam scanning optical system according to the related art. FIG. 11 is an explanatory diagram showing a space between two light beams in a sub-scanning direction (a rotation axis direction of a deflector 7 which will be described later on). FIG. 12 is an explanatory diagram showing the manner in which the space between the two light beams in the sub-scanning direction is measured by a sensor unit. FIG. 13 is a diagram of a waveform of an output signal from the sensor unit. In FIGS. 10 to 12, first and second light sources 1a, 1b emit light beams, respectively. Collimator lenses 2a, 2b collimate the light beams from the first and second light sources 1a, 1b to provide substantially parallel light beams, respectively. A prism 3 is provided to adjust a light path of a sub-scanning direction of the light beam from the second light source 1b. An adjustment mechanism 4 is provided to adjust a light path of an light beam by rotating the prism 3. A beam splitter 5 is provided to match optical axes of light beams emitted from the first and second light sources 1a, 1b. A cylindrical lens 6 is provided to focus the sub-scanning direction of the light beam emitted from the beam splitter 5. A deflector 7 has a deflection surface disposed near the focus of the cylindrical lens 6 to deflect two light beams simultaneously. A scanning lens 8 is provided to focus and scan the two light beams deflected by the deflector 7. A detection unit 9 is provided to detect a space between the two light beams in the sub-scanning direction. A synchronizing detector 10 is disposed on non-image areas of scanned light beams to synchronize the scanned light beams. Reference numeral 11 denotes a mirror for introducing light beams into a scanned surface 14. Reference numeral 12 denotes a housing. Reference numeral 13a denotes a scanned locus formed by the light beam emitted from the light source 1a. Reference numeral 13b denotes a scanned locus formed by the light beam emitted from the light source 1b. Reference numeral 15a denotes a driving circuit for driving the light source 1a. Reference numeral 15b denotes a driving circuit for driving the light source 1b. Reference numeral 16 denotes a sensor disposed in the detection unit. Reference numeral 17 denotes a deflection plane of the deflector 7.
An operation of the two-light-beam scanning optical system thus arranged according to the related art will be described below.
As shown in FIG. 10, light beams emitted from the first and second light sources 1a, 1b are collimated by the collimator lenses 2a, 2b into substantially parallel light beams. With respect to the collimated two light beams, in order to match a space between the collimated two light beams with a predetermined space, a light path of the other light beam (light beam from the first light source 1b) is adjusted by the prism 3 relative to a light path of the reference light beam (light beam from the first light source 1a). The two light beams are introduced into the beam splitter 5, in which the optical axes are substantially matched and focused in the sub-scanning direction by the cylindrical lens 6. Sine the deflection surface 17 of the deflector 7 has a very slight inclination (hereinafter this inclination will be referred to as "surface inclination") in the vertical direction of each surface, an optical conjugate relationship is established between the deflection surface 17 and the scanned surface 14 by the scanning lens 8, thereby resulting in an influence of surface inclination being alleviated.
The sub-scanning direction space of the two light beams on the deflection surface 17 affects the sub-scanning direction space on the scanned surface 14 in response to a sub-scanning direction magnification of the scanning lens 8. Accordingly, it is possible to change the space between the two light beams on the scanned surface 14 by changing the sub-scanning direction space of the two light beams incident on the deflection surface 17. As shown in FIG. 11, by rotating the prism 3 relative to the reference light beam in the sub-scanning direction, the two-light-beam space p on the deflection surface 17 is set to a predetermined space. When the prism 3 is controlled, the detection unit 9 disposed near the scanned surface 14 detects the sub-scanning direction space of the two beams, and stops the rotation of the prism 3 at the position in which the predetermined space can be achieved.
FIGS. 12 and 13 show the manner in which the sub-scanning direction space of the two light beams is measured. In FIG. 12, the sensor 16 of the detection unit 9 (FIG. 10) is a sensor having a V-shaped light-receiving surface which emits a different output signal depending upon the position of a scanned light beam. As shown in FIG. 13, for example, the output signal of the light beam which scans the upper portion of the sensor 16 has an interval longer than that of the output signal of the light beam that scans the lower portion of the sensor 16. By controlling the prism 3 through the V-shaped sensor 16, the sub-scanning direction space can be matched with the predetermined space.
In FIG. 10, although the light beam incident on the deflector 7 is deflected and focused on the scanned surface 14, the light path of the scanning optical system is generally introduced by the mirror 11 into the scanned surface 14 in order to make the optical system compact. The two light beams that are scanned by the deflector 7 are synchronized with each other in the sub-scanning direction by the synchronizing detector 10 located ahead of the position at which the image area is scanned, thereby resulting in light beams corresponding to image data being irradiated at a timing of a predetermined time. Since the scanned surface 14 is moved in the sub-scanning direction (in the direction perpendicular to the main scanning direction in which light beams are scanned), an image can be formed on the scanned surface 14 by two-dimensional light irradiation. With respect to the space between the two light beams, an interval between generated signals is changed depending upon the height in which the light beam scans. If a light path is corrected in such a manner that this signal interval becomes a predetermined interval, then it is possible to adjust the sub-scanning direction space. In this manner, the image can be formed on the scanned surface 14 by two light beams, whereby there is provided a two-light-beam scanning optical system of high speed or high resolution.
However, in the above-mentioned two-light-beam scanning optical system according to the related art, since the position of the light beam is detected by using scanned light beams, when the scanning speed is high, a quantity of light is reduced so that the level of the output signal is lowered. Moreover, since the sensing time is reduced, a detection accuracy of light beam is lowered. Further, since the position of the scanned light beam is fluctuated due to the surface inclination of the deflector 7, there is a problem that it is difficult to determine the sub-scanning direction space.
In addition to the above-mentioned system according to the related art, there is proposed a method such that a light path is corrected by detecting the beam space of the sub-scanning direction by using the light beams from the beam splitter 5 which makes light paths coincident with each other. In this case, there are arrayed two bisector sensors. When outputs from the two bisector sensors become equal to each other in level, the beam position is determined. There is then the problem that the space between the light-receiving surfaces of the bisector sensors is to be formed with a high accuracy.
Furthermore, when the light path of the sub-scanning direction space is corrected in the two-light-beam scanning optical system according to the related art, the angle of the light path is changed by using the prism 3 or the like so that the positions at which two light beams pass the scanning lens 8 become different. As a result, the scanning speed and the curvature of field on the scanned surface 14 become different. There is then the problem that an image is deteriorated.
In the two-light-beam scanning optical system, it is requested to improve the detection accuracy in the sub-scanning directions of two light beams. Also, it is requested to form an image of a high quality by eliminating errors of scanning speed and curvature of field on the scanned surface of two light beams.
Therefore, the present invention is to provide a two-light-beam scanning optical system in which the detection accuracy in the sub-scanning directions of two light beams can be improved and in which an image of a high quality can be improved by eliminating errors of scanning speed and curvature of field on the scanned surface of two light beams.